C# trains readers to expect strong tooling, namespaces, nullable annotations, records, interfaces, LINQ, tasks, and attributes. Musi uses similar clarity but different boxes: records/data hold shape, classes and instances hold behavior, Option names absence, and effects make async or runtime work visible instead of hiding it behind a task-returning method.
Use this guide as a translation journal for C# code. Start with the familiar habit, then ask which Musi construct owns the same promise. A page is successful when it tells you which old instinct to keep and which old shortcut to drop.
Start from the habit that hurts
- Values, Methods, and Expressions - values habits get translated once, then the Musi Book owns the details.
- Blocks and Control Flow - blocks habits get translated once, then the Musi Book owns the details.
- Variables and Mutation - mutation habits get translated once, then the Musi Book owns the details.
- Records, Classes, and Objects - records habits get translated once, then the Musi Book owns the details.
- Collections, LINQ, and Pipelines - arrays habits get translated once, then the Musi Book owns the details.
- Null, Option, and Result - absence and failure stop being side channels.
- Exceptions, Results, and Effects - outside work becomes a named request.
- Unions and Pattern Matching - patterns habits get translated once, then the Musi Book owns the details.
- Generics, Interfaces, and Laws - generics habits get translated once, then the Musi Book owns the details.
- Extension Methods and Calls - calls habits get translated once, then the Musi Book owns the details.
- Namespaces, Modules, and Packages - mutation habits get translated once, then the Musi Book owns the details.
- Testing and Tooling - testing habits get translated once, then the Musi Book owns the details.
- Unsafe, Interop, and FFI - native edges stay narrow enough to audit.
Habits that still help
- Keep the C# habit of making API shape readable at the call site.
- Drop the assumption that every reusable behavior needs an object hierarchy or service object.
- Keep examples tied to ordinary work: receipts, routes, files, animals, people, rooms, and services.
First false friend
Musi class is a typeclass-style behavior contract, not a CLR class with fields, constructors, and inheritance. If you need a person, car, invoice, or package, start with records or data. If you need any type that can compare, show, encode, or step, use a class with instances.
When to switch to the Musi Book
Stay in this guide while a C# habit is getting in the way. Switch to Values and let, Records, Patterns, Generics, Classes, and Effects when you need the source-of-truth rule.